Project Summary Paclitaxel-induced peripheral neuropathy (PIPN) occurs in up to 80% of patients received this therapy. PIPN is characterized by distal limb neuropathic pain and loss of intraepidermal nerve fibers (IENF). Currently there is no effective treatment for this debilitating neuropathic pain syndrome. We recently found that complement, a key component of innate immunity, plays a previously unknown yet pivotal role in the development of PIPN. Complement activation leads to 1) release of anaphylatoxins C3a and C5a to mediate inflammation through their receptors C3aR and C5aR, and 2) formation of membrane attack complex (MAC) to mediate cell damage. Because rats are superior to mice for many behavioral tests, including pain measurement, we created the first- ever complement knockout (KO) rats lacking C3, the central component of the complement activation cascade. Our preliminary studies showed that Paclitaxel causes complement activation in vitro and in vivo. Paclitaxel- induced mechanical allodynia (PIMA, a readout of pain), loss of IENF, deposition of MAC, and expression of transient receptor potential vanilloid 4 (TRPV4, a key mechanosensory receptor for pain) in dorsal root ganglion (DRG) are much less in C3 KO rats than in WT controls. Others have reported that macrophage infiltration by CCL2/CCR2 chemoattractant signals into DRG and intracellular calcium influx in DRG neurons play essential roles in PIPN. Based on these findings, we hypothesize that complement activation (i) stimulates DRG neurons to produce CCL2 to subsequently recruit macrophages into DRG; (ii) upregulates TRPV4 expression and stimulates Ca2+ influx in DRG through C3aR/C5aR activation; and/or (iii) directly damages DRG neurons through MAC formation. Thus, inhibiting complement should prevent/treat PIPN. We will first determine the role of complement in paclitaxel-mediated CCL2/CCR2 activation, macrophage infiltration into DRG, as well as the therapeutic effect of C3aR/C5aR antagonists on PIMA in PIPN (Aim 1). We will then examine the role of complement in paclitaxel-mediated TRPV4 upregulation, intracellular Ca2+ influx, and MAC formation in DRG neurons (Aim 2). Finally, we will correlate complement activation and loss of IENF in patients developed PIPN (Aim 3). This project will, for the first time, elucidate the mechanisms by which complement contributes to the development of PIPN. Our work might also provide a solid foundation for the future development of complement-targeted therapeutics to manage PIPN that has no effective treatment yet. Because other peripheral neuropathies (e.g., due to alcohol consumption, HIV infection or Diabetes Mellitus) are also characterized by distal neuropathic pain and loss of IENF, findings of this study would have broad clinical impact in managing these hard-to-treat pain syndromes.